High speed holographic optical printing system

ABSTRACT

An optical system is disclosed which, when utilized in conjunction with an electrographic printer and a laser light source, provides high quality characters at very high print speeds with a minimum of laser power. Horizontal and vertical light beam deflectors are employed to access a hologram in a two dimensional holographic array. The horizontal deflector is controlled by internally generated timing pulses to sequentially step from one column of the array to another, each column corresponding to a printing position on a line of print, while the vertical deflector is controlled by an external electrical signal to position a spatially unmodulated laser beam to illuminate the desired hologram in the column. The illuminated hologram emits a spatially modulated beam that forms an image along a line of print, at a position on a conductive drum corresponding to the column of the illuminated hologram.

This is a continuation of application Ser. No. 810,245, filed June 27,1977, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical printing systems and more particularlyto a high speed optical printing system employing a two dimensionalholographic array, a hologram element of which is accessed by aspatially unmodulated substantially collimated light beam and inresponse thereto emits a spatially modulated beam which forms an imageon a photoconductive drum.

2. Description of the Prior Art

Standard impact printer systems employ a rotating or translating memberthat contains one or more sets of the character vocabulary. To accessthe proper character an instant of time is selected, from the totalavailable cycle time, at which the impact of a hammer transfers thedesired character from the rotating of translating member to theprintout paper. This approach has been employed to produce an opticalprinter wherein a light beam is directed, at the proper time, to a maskcontaining transparencies or templates of the character vocabulary toilluminate the desired character contained therein. The spatiallymodulated beam resulting therefrom is focussed upon a photoconductivedrum member of an electrophotographic copier system wherein thecharacter is rendered visible and transferred to a hard copy paperimage. This time domain accessing of characters requires the light beam,which illuminates the rotating character mask, to be pulsed not only atthe proper time to select the desired character but in such a fashion asto hold character blurring within acceptable tolerances. Consequently,the average light pulse duty cycle must include a pulse that issufficiently brief to provide an instantaneous snapshot of the selectedcharacter at an average interpulse period that is equivalent to onecycle of the entire vocabulary of characters. To prevent blurring, thecharacter motion must be less than five percent of the characterdimension. Thus, if the printable vocabulary contains 100 symbols, anaverage light pulse duty cycle of 5×10⁻⁴ is required. This duty cycledictates a peak power level for the pulse system's light source that is2,000 times greater than that of a light source in an illuminationsystem which could function with a 100 percent duty cycle.

Time domain accessing for optical printing systems as described aboveexhibit printing speed limitations. A system employing a character reelthat rotates at 3600 rpm and carries two vocabulary sets on itscircumference has a printing rate of 120 characters per second orapproximately 60 lines of printing per minute. To surmount this printingspeed limitation a multiplicity of light sources has been employed andprinting speeds of several thousand lines per minute have been achievedby employing one light source for each printing line on a page. This isa brute force approach that is expensive and which results in a shortmean time between failures (MTBF). It will readily be appreciated thatprinting speeds of optical printing systems may be increased, withoutcharacter blurring, by replacing the time dimension accessing procedurewith a system that randomly positions a beam to access a stationaryarray of character generating masks.

SUMMARY OF THE INVENTION

The subject invention is a high speed printing system which includes alaser light source for illuminating a two dimensional light beamdeflection system wherefrom the light beam from the laser source isdeflected to access a desired hologram positioned in a two dimensionalarray of holograms. A spatially modulated light beam is directed fromthe illuminated hologram to form an image at a desired line position ona photoconductive drum. All columns of the two dimensional array ofholograms are identical, each containing an ordered linear array ofhologram elements each of which generates one character of the printedvocabulary. Each character position on a line of print is sequentiallyaccessed by horizontally deflecting the light beam through one columnposition while simultaneously accessing the desired character byvertically deflecting the beam to the proper row, thus internallygenerated timing pulses readily control the printing rate and nointernal buffer storage of character is required. Additionally,non-stepping motion of the electrophotographic drum may be achieved byhaving the hologram elements in succeeding columns provide images on aline that is slightly slanted to compensate for the continuous motion ofthe photoconductive drum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical schematic diagram of an embodiment of theinvention.

FIG. 2 is a cross-sectional view of the holographic array diffractiongeometry.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a high speed holographic optical printer 10includes a laser source 11, such as a 441 nm He-Cd or a 488 nm argon,that emits a substantially collimated spatially unmodulated, light beam12 which enters a two dimensional deflection system 13, wherefrom alight beam 14 is emitted to illuminate a selected hologram 15 positionedin a two dimensional array of holograms 16. A spatially modulated lightbeam 17 is emitted from the illuminated hologram 15 to form an image 18at a preselected position on a photoconductive drum 19 which may be ofthe selenium type. The optical system shown in FIG. 1 provides formaximum printing speed by minimizing the optical information that isprocessed by the two dimensional deflection system 13. The spatiallyunmodulated light beam 12 emitted from the laser source 11 is deflectedby the deflection system 13 to access the hologram 15, whereat thespatial modulation is imparted to the light beam which provides thecharacter information to form an image on the continuously rotatingphotoconductive drum 19. The deflector system 13, thus provides as manybeam positions as required to address all the holograms in theholographic array 16 while processing only one resolvable beam positionat a time, thereby providing the highest possible access speed.

The holographic array 16 comprises a two dimensional array of individualholograms, each of which converts a substantially collimated, spatiallyunmodulated, light input signal into a spatially modulated light beamfrom which an image of an alpha-numeric character is formed at a givenposition on the photoconductor drum 19. The columns 20 of the array 16are identical, each containing an ordered linear array of hologramelements 21 each of which generates one character of the printedvocabulary. Hologram elements 21, in each column 20 of the holograms,are recorded in such a fashion that the focussed image of the charactergenerated by a hologram element 21 will be produced at a position,corresponding to the column 20, along a common line 22 on thephotoconductive drum 19 regardless of the location of the hologramelement 21 in the column 20. Stepping the horizontal position of thedeflected light beam 14 to the adjacent column 20 accesses the adjacentposition on the common line 22 and changing the vertical position of thedeflected light beam 14 provides for accessing the hologram element 21corresponding to the subsequent character to be printed.

Each elemental hologram 21 taken from a single column 20 includes both apredetermined interference fringe spacing which diffracts the inputlight beam into the correct direction toward the common line 22 positionas well as a coded spatial message which generates a remotely focussedprinted character onto the surface of a photoconductive drum. Theholographic array 16, while acting as a cylindrical lens in onedimension to focus distinct characters onto the common line 22 containsno such focussing action in the orthogonal direction inasmuch as eachcolumn 20 of hologram elements 21 must provide access to separate printpositions on the photoconductive drum 19.

Maximum sharpness and quality of the characters produced by eachhologram element 21 of the holographic array 16 are limited by lightdiffraction effects. Consequently, a hologram element with an edgedimension A that is illuminated by a substantially collimated light beamhaving a wavelength λ, and positioned a distance D from thephotoconductive drum 19 will provide a linear resolution on thephotoconductive drum 19 of ΔS=Dλ/A. A minimum distance between anyhologram element in the holographic array 16 and the photoconductivedrum 19 is determined by a fall off in hologram diffraction efficiencyat large angles. Since the diffraction efficiency of standardphotographic films is reasonably constant up to 60 degrees, a possiblediffraction geometry may be that as shown in vertical cross-section inFIG. 2 wherein D_(m) represents the minimum distance between a hologramelement and the photoconductive drum 19. If all dimensions are incentimeters, the maximum image resolution on the photoconductive drum 19is then given by A/D_(m) λ lines/cm.

The holographic array 16 arrangement described above minimizes thepositional deflection accuracy required from the two dimensionaldeflection system 13. Each position of the printed character is solely afunction of the position and focussing of the hologram element 21,which, when substantially illuminated, causes a character to be producedon the common line 22 at the location associated therewith.Additionally, the holographic array 16 provides a higher lightthroughput efficiency than standard template mask imaging techniques.Theoretically, the light throughput efficiency of a hologram can be 100percent. In practice, holograms have been fabricated which diffract inexcess of 25 percent of the incident light beam into a remotely focussedhigh quality image of the recorded character. Conversely, template maskstypically transmit only a few percent of the incident light.Consequently, the use of holographic arrays either increases the speedcapability of an optical printer system or lowers the laser powerrequired for a comparable printing speed. Further, the hologram elements21 in each column 20 of the holographic array 16 may be made tosequentially print characters along a slanted print line to compensatefor the motion of the photoconductive drum. The time delay in printingone line can then be used in part to create the proper vertical spacingbetween adjacent lines. Continuous motion of the drum withoutcomplicated electronic pulsing to compensate for vertical charactermispositioning as would occur in time domain character accessing offersa significant simplification in mechanical structure and results in anet cost savings.

Random positioning of the laser beam to access a hologram element 21allows the laser source 11 write time to be greater than the light beam12 positioning time. Consequently, as opposed to systems employingrotating members and time domain accessing, the effective laser dutycycle may approach unity which provides for a more efficient utilizationof the laser source.

The two dimensional light beam deflection system 13 may comprise avertical light beam deflector 27 and a horizontal light beam deflector28. A substantially collimated light beam 12, incident to the verticallight beam deflector 27 is deflected therefrom at the proper verticalposition to access a selected hologram element 21. This deflected lightbeam is then incident to the horizontal light beam deflector 28 whereat,it is deflected at the proper horizontal position to illuminate theselected hologram element 21 in the column 20 corresponding to theposition on the line 22 at which the character is to be printed.Deflectors 27 and 28 must be capable of rapid random access positioning.Mechanical galvanometer deflectors such as the Honeywell M25K areavailable which can randomly access any of 220 Raleigh resolved beampositions in 50 microseconds and acousto-optic deflectors, areavailable, such as those manufactured by Isomet and Soro (French), whichcan access any of 400 to 1,000 resolvable positions in 5 microseconds.These random access speeds provide a maximum limiting printer speed thatis approximately 10,000 lines per minute (LPM) for galvanometerdeflectors and 100,000 LPM for acousto-optic deflectors. Since theholographic array 16 is comprised of m rows and n columns, m×n hologramelements 21 must be accessed. For the light beam deflector system 13 toseparately access each hologram element 21, it must be capable ofdeflecting the substantially collimated light beam 12 to approximatelytwice the number of resolvable beam positions in each dimension.Consequently, the vertical light beam deflector 27 must be capable ofproviding 2m resolvable beam positions and the horizontal light beamdeflector 28 must be capable of providing 2n resolvable beam positions.The two dimensional light beam deflector 13 may also include a focussinglens 29 placed between the vertical deflector 27 and the horizontaldeflector 28 to refocus light deflected from deflector 27 onto deflector28 and a lens 30 which functions to ensure the collimation of the lightbeam 14 that is emitted from the horizontal deflector 28 to provide amaximum number of resolvable beam positions at the holographic array 16.

The utilization of a single light source requires each characterposition on a line to be accessed by horizontally deflecting the lightbeam sequentially from one column position to another and for eachcolumn position the correct character is accessed by deflecting the beamthrough an arbitrary number of vertical positions. This electricaladdressing may be simplified by having the input signal code onlycontrol the vertical positioning of the light beam deflector 27 andhaving internally generated timing pulses, which determine the printingand line rates, control the horizontal position of the horizontal lightbeam deflector 28. The internally generated timing pulses would advancethe printer character position automatically one increment after eachcontrol signal input. In FIG. 1, an initial vertical light beamdeflection and a subsequent horizontal light beam deflection isindicated. It will be apparent to those skilled in the art that thesemay be reversed without adversely affecting the over-all operation ofthe system.

While the invention has been described in its preferred embodiment, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes may be made withinthe purview of the appended claims without departing from the true scopeand spirit of the invention in its broader aspects.

I claim:
 1. An apparatus for projecting characters along recording linesof a continuously rotating photoconductive drum comprising:an array ofcharacter generating elements wherein each column of elementscorresponds to a preassigned position on said recording lines whereat acharacter is formed when an element in said column is illuminated by asubstantially collimated light beam, said array constructed and arrangedsuch that said character generating elements in each column arepositioned to provide characters along a slanted print line to providecompensation for said continuous rotation of said photoconductive drumthereby establishing non-slanted recording lines; means for directing alight beam to be incident to a selected element in a selected column;and means positioned between said directing means and said array ofimage generating elements for substantially collimating said incidentlight beam.
 2. An apparatus in accordance with claim 1 wherein saiddirecting means includes:first means for deflecting a light beamincident thereto in a predetermined plane; and second means fordeflecting a light beam incident thereto from said first deflector meansin a plane perpendicular to said predetermined plane, said light beamdeflected from said second deflecting means being directed along a pathto illuminate a preselected element in said array of charactergenerating elements.
 3. An apparatus in accordance with claim 2 whereinsaid first and second deflecting means are constructed and arranged suchthat said columns are sequentially accessed at a predetermined rate anda selected one of said character generating elements in an accessedcolumn is selected in a random fashion to provide a selected characterat said corresponding position on said recording line.
 4. An apparatusin accordance with claim 2 further including:means positioned betweensaid first and second deflecting means for focussing light beamsdeflected from said first deflecting means onto said second deflectingmeans; and means positioned between said second deflecting means andsaid array of character generating elements for emitting a substantiallycollimated light beam to illuminate said preselected charactergenerating element.
 5. An apparatus in accordance with claims 1, 2, 3 or4 wherein said elements in each of said columns are located at at apreselected minimum distance from said rotating photoconductive drum andarranged to provide diffracted beams within an angular range between 20°and 60° from said incident light beam.